Mixing and internal dynamics of a medium-size and deep lake near the Arctic Circle

Abstract

The goal of the research was to identify the mixing features that distinguish wind forced, medium-size fjord- type lakes near the Arctic Circle from systems of similar geometry, but in a temperate climate. Experimental data and the results of 3D numerical hydrodynamic simulations were analyzed for Lake Lagarfljót (27 km long; 2 km wide; 110 m maximum depth; 65 N) during the 5-month ice-free period in 2009. The results showed that prob- ably the most distinctive feature of arctic lakes is their low water column stability, one order of magnitude lower than that of mid-latitude lakes, even in mid-summer. The second characteristic is the high rate of energy imparted by wind in the Arctic, estimated as one order of magnitude larger than in a temperate climate. Frequent wind events with Lake and Wedderburn numbers below 1 occurred during the ice-free period, leading to a strong shear at the base of the surface mixed-layer and the upwelling of deep metalimnetic layers. As a result, nearly continuous density stratification developed in the shallowest 70 m of the water column, and lon- gitudinal temperature gradients may therefore be very significant. On average, 15% of the wind forcing drove large-scale internal motions. This energy was rapidly dissipated, partly as a result of stronger bottom velocities, nonlinear surges, and hydraulic jumps that repeatedly formed in the lake. The initial V1H1 setup rapidly evolved into spatially complex oscillatory modes, which, given the high latitude, are affected by the Earth’s rotation, even in narrow basins.